Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating a mirror over the torsion beams. The elevated mirror blocks incident light from striking the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support. These support structures, along with the address electrodes and mirror bias/reset metalization on the device substrate, all tend to scatter light striking them. This scattered light reaches the image screen and reduces the contrast ratio of the projected image. The hidden-hinge micromirror design improves the contrast ratio of the images by preventing most light from reaching these support structures.
Elevating the micromirror above the torsion beams and their supports requires a support structure to space the micromirror above the torsion beams. Typically, a spacervia or support post is fabricated to perform this task. A spacervia is a hollow tube of metal formed by depositing metal into a hole in the sacrificial layer on which the mirror is fabricated. The hollow spacervia has an open top which decreases the surface area of the micromirror. Additionally, the open top of the spacervia is a sharp edge which diffracts incident light--lowering the contrast ratio of the projected image.
In addition to the need to improve the contrast ratio of the projected images, micromirror designers also strive to improve the reliability of the mirror reset--the act of returning the micromirror to a neutral position after it has been rotated to either the on or off position. Some micromirrors tend to stick to the landing sites due to a variety of forces such as the van der Waals force generated by water vapor present on the device surface and intermetallic bonding. A technique called dynamic reset uses voltage pulses to excite the dynamic response of the micromirror and torsion beams causing the mirror to spring away from the landing site and back to a neutral position.
Unfortunately, the magnitude of the force sticking the mirrors to the landing sites varies over a wide range. Mirrors that are stuck to the landing sites only weakly often release from the landing site after a single reset pulse, while other mirrors may require several pulses before storing sufficient energy to release from the landing site. Prematurely released mirrors cause several problems. First, a prematurely released micromirror may land during the remainder of the dynamic reset period. If the prematurely released micromirror lands late enough in the dynamic reset period, the mirror will not be able to store enough energy from the reset pulses to release from the landing site a second time. Second, the prematurely released micromirror may tend to flutter about the axis of the torsion beams. If the mirror bias voltage is reapplied while the fluttering mirror is rotated toward the wrong address electrode, the mirror will be electrostatically latched to the wrong address electrode--causing an intermittent twinkling of dark pixels.
Therefore, there is a need for a micromirror device design which improves the contrast ratio of the projected image and also improves the reliability of the micromirror reset operation.